The photographic objective lens simulating fisheyes, which is also referred to “fisheye” photographic objective lens, is a photographic objective lens with a large relative aperture. According to the optical theory, the field of view of the photographic objective lens can be 180°, i.e. all sceneries in front of the photographic objective are captured in the field of view of the objective lens.
Assuming that the sceneries in front of the semi-spherical surface of the objective lens are totally captured and imaged on the image plane, it can be inferred that, the object space in front of the objective lens is a semi-spherical space while the image surface is a circular surface. At this time, the formula η=ftgα in the geometric optics is no longer applicable, because when α in tgα is 90°, tgα→∞, i.e. η→∞, which is impossible in practice. Then the practical optical system distortion value of the optical theory of the objective lens is 100%, in which η≈f, such that the focal length f of the optical system determines that diameter of the image of the object surface should be 2f, such a photographic objective lens usually has a great volume.